Warm Roof Air Gap

The amount of ventilation or air change needed can be calculated by.

Warm roof air gap.

Ventilation air follows exactly the opposite path. Purlins at a 45 degree angle still provide sufficient support for installation of the metal roof panels. An unvented roof assembly is possible only if you keep the roof sheathing warm enough to prevent conditioned air from condensing against it. A warm roof uses solid pir insulation boards that sit above the roof deck topped by moisture tolerant boards.

Ventilation is not needed in a warm roof. A sealed warm roof construction can also be achieved by creating a 50mm ventilated air gap between the surface of the insulation and the undertile lining to allow water vapour to be safely removed from the roof construction. A cold roof is where all of the insulation is placed either between and under or completely under the joists or deck. Rising along the underside of the angled purlin until it reaches a gap then rising up to the next purlin and so on to exit at the ridge or gable.

The map at right which is based on table r806 4 of the irc lists the minimum r values required to prevent condensation in unvented assemblies in various climate zones. There should be a ventilation gap created to allow air from the outside to be drawn into this space.